Good plan, that is what I usually end up doing. Every time I have ever been in Home Depot fitting up pieces for something atypical like a woofer port, a well intentioned employee comes along and asks if they can help. Blank stare is usually the reaction when I try to explain.

ABS is the black pipe. PVC will work as well, but it is more expensive and tougher to find in that size.

Sorry, I haven't had much time to ponder this. I will look at the manual tonight and work out a method to get you close.

An added complication is how the system is wired. Does your receiver have pre-outs/main ins for the front channels? If not you are probably stuck with using the passive high pass built into the sub amp. That requires running the front channel speaker wires to the sub amp, and then from the output to the speakers. The sub signal will be pulled from the incoming speaker signal.

I would start by turning the sub's crossover down as far as it can go. With that setting, use the signal generator to get a feel for your sub's response curve, finding the peak, seeing how flat it runs, etc. Then, match the peak/flat area to your mains, using your signal generator to generate tones for you to test. The biggest advice I can give in this area is that by ear, you hear a lot less bass than you do higher frequencies, so in my experience, the bass should be softer than the higher frequencies or you end up with bloated bass.

Find the frequency where your main speakers start to roll off significantly. Run the crossover control up and listen for when your sub starts to fill in the gap. If it sounds like there's a region in the bass where no matter how much you turn up the crossover, and even to a limited extent turn up the volume on the sub, it just doesn't get much louder and it sounds like there are some weird cancellation effects, adjust the phase control. I would match it first by getting the crossover up roughly to where it starts filling in the bass left out by your mains. Then match the phase. It should be loudest when the sub is in phase. Then carefully match the volume of the sub to your mains adjusting the crossover as needed. You may get different results with the new phase setting.

So basically:
0. turn your sub's crossover all the way down
1. locate where your mains fall off
2. bump your subwoofer's crossover freq up to roughly where it matches
3. match phase by listening to various frequencies and seeing if there are nulls being created by phase mismatch
4. fine tune your crossover frequency and overall volume of the sub

ok, I am going to give you 2 more techniques. I actually have a couple more yet, but they involve test gear or the potential of smoke coming out of your gear, and I would not try to walk someone through either of those via text.

The first one works best with an assistant, since the controls are at the sub, which is presumably not your listening position. I also advise that you use the sub amp's internal high pass on the satellites (front channels). One of the benefits of a sub is to reduce cone excursion in the satellites, as well as power requirements to same. The built in high pass is set at 125 hz for your speakers.

Set the sub level to zero, and the crossover to the highest setting (160 hz).

Feed a 150 hz signal to the system (to an input into the receiver).

Turn the volume control on your receiver up until the tone is at a clear level at your listening position.

Have your assistant turn the sub level up until you hear an audible difference.

Have the assistant turn the phase knob back and forth until the level peaks at your listening position.

If the level gets too loud, you may have to turn the sub down all the way, then bring it back up until you start hearing its contribution. The idea is to have the sub and sats at roughly equal levels, and if you start out completely out of phase, the sub will dominate too much.

When you find the peak, the sub and sats are in phase.

Set the sub crossover to 125 hz. You may want to drop it lower because the room may have a mode around there, and the sat slope is only first order, so they won't go away all that fast as frequency drops.

As for the level, that is tougher in this configuration. Basically the bass should be at the same level as the rest of the music. Without a test mic you just have to tweak by ear. Everybody starts out with the sub set too high, and some back it down over time. It should sound musical, not dominant. Unfortunately you will get room modes that will complicate matters, but that is life in the great indoors.

The second technique requires measurement software and a mic. If you get interested enough to purchase a more detailed version of TrueRTA, or find a freebie equivalent, I can tell you how to dial things in using even a PC mic and soundcard. The measured response won't be accurate, but it will tell you what is happening at the crossover point.